| 1. |
- Roelfsema, P. R., et al.
(författare)
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In-orbit performance of Herschel-HIFI
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 537
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Tidskriftsartikel (refereegranskat)abstract
- Aims: In this paper the calibration and in-orbit performance of the Heterodyne Instrument for the Far-Infrared (HIFI) is described.Methods: The calibration of HIFI is based on a combination of ground and in-flight tests. Dedicated ground tests to determine those instrument parameters that can only be measured accurately using controlled laboratory stimuli were carried out in the instrument level test (ILT) campaign. Special in-flight tests during the commissioning phase (CoP) and performance verification (PV) allowed the determination of the remaining instrument parameters. The various instrument observing modes, as specified in astronomical observation templates (AOTs), were validated in parallel during PV by observing selected celestial sources.Results: The initial calibration and in-orbit performance of HIFI has been established. A first estimate of the calibration budget is given. The overall in-flight instrument performance agrees with the original specification. Issues remain at only a few frequencies. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
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| 2. |
- Decin, L., et al.
(författare)
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Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen by Herschel/HIFI
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L4-
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Tidskriftsartikel (refereegranskat)abstract
- During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the high-resolution HIFI spectrometer onboard Herschel. We report on the first detection of (H2O)-O-16 and the rarer isotopologues (H2O)-O-17 and (H2O)-O-18 in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 x 10(-5), and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H2O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of (CO)-C-12, (CO)-C-13, (SiO)-Si-28, (SiO)-Si-29, (SiO)-Si-30, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.
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| 3. |
- Alcolea, J., et al.
(författare)
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HIFISTARS Herschel/HIFI observations of VY Canis Majoris. Molecular-line inventory of the envelope around the largest known star
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 559, s. (article nr.) A93-
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Tidskriftsartikel (refereegranskat)abstract
- Aims.The study of the molecular gas in the circumstellar envelopes of evolved stars is normally undertaken by observing lines ofCO (and other species) in the millimetre-wave domain. In general, the excitation requirements of the observed lines are low at thesewavelengths, and therefore these observations predominantly probe the cold outer envelope while studying the warm inner regions ofthe envelopes normally requires sub-millimetre (sub-mm) and far-infrared (FIR) observational data.Methods.To gain insight into the physical conditions and kinematics of the warm (100–1000 K) gas around the red hyper-giantVY CMa, we performed sensitive high spectral resolution observations of molecular lines in the sub-mm/FIR using the HIFI instru-ment of theHerschelSpace Observatory. We observed CO, H2O, and other molecular species, sampling excitation energies from afew tens to a few thousand K. These observations are part of theHerschelguaranteed time key program HIFISTARS.Results.We detected theJ=6–5,J=10–9, andJ=16–15 lines of12CO and13CO at∼100, 300, and 750 K above the groundstate (and the13COJ=9–8 line). These lines are crucial for improving the modelling of the internal layers of the envelope aroundVY CMa. We also detected 27 lines of H2O and its isotopomers, and 96 lines of species such as NH3,SiO,SO,SO2HCN, OH andothers, some of them originating from vibrationally excited levels. Three lines were not unambiguously assigned.Conclusions.Our observations confirm that VY CMa’s envelope must consist of two or more detached components. The molecularexcitation in the outer layers is significantly lower than in the inner ones, resulting in strong self-absorbed profiles in molecular linesthat are optically thick in this outer envelope, for instance, low-lying lines of H2O. Except for the most abundant species, CO and H2O,most of the molecular emission detected at these sub-mm/FIR wavelengths arise from the central parts of the envelope. The spectrumof VY CMa is very prominent in vibrationally excited lines, which are caused by the strong IR pumping present in the central regions.Compared with envelopes of other massive evolved stars, VY CMa’s emission is particularly strong in these vibrationally excitedlines, as well as in the emission from less abundant species such as H13CN, SO, and NH3.
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| 4. |
- Cernicharo, J., et al.
(författare)
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A high-resolution line survey of IRC+10216 with Herschel/HIFI First results: Detection of warm silicon dicarbide (SiC2)
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L8-
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Tidskriftsartikel (refereegranskat)abstract
- We present the first results of a high-spectral-resolution survey of the carbon-rich evolved star IRC+10216 that was carried out with the HIFI spectrometer onboard Herschel. This survey covers all HIFI bands, with a spectral range from 488 to 1901 GHz. In this letter we focus on the band-1b spectrum, in a spectral range 554.5-636.5 GHz, where we identified 130 spectral features with intensities above 0.03 K and a signal-to-noise ratio >5. Detected lines arise from HCN, SiO, SiS, CS, CO, metal-bearing species and, surprisingly, silicon dicarbide (SiC2). We identified 55 SiC2 transitions involving energy levels between 300 and 900 K. By analysing these rotational lines, we conclude that SiC2 is produced in the inner dust formation zone, with an abundance of similar to 2 x 10(-7) relative to molecular hydrogen. These SiC2 lines have been observed for the first time in space and have been used to derive an SiC2 rotational temperature of similar to 204 K and a source-averaged column density of similar to 6.4 x 10(15) cm(-2). Furthermore, the high quality of the HIFI data set was used to improve the spectroscopic rotational constants of SiC2.
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| 5. |
- De Beck, Elvire, 1985, et al.
(författare)
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Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. II. CO line survey of evolved stars: derivation of mass-loss rate formulae
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 523
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Tidskriftsartikel (refereegranskat)abstract
- Context. The evolution of intermediate and low-mass stars on the asymptotic giant branch is dominated by their strong dust-driven winds. More massive stars evolve into red supergiants with a similar envelope structure and strong wind. These stellar winds are a prime source for the chemical enrichment of the interstellar medium. Aims: We aim to (1) set up simple and general analytical expressions to estimate mass-loss rates of evolved stars, and (2) from those calculate estimates for the mass-loss rates of the asymptotic giant branch, red supergiant, and yellow hypergiant stars in our galactic sample. Methods: The rotationally excited lines of carbon monoxide (CO) are a classic and very robust diagnostic in the study of circumstellar envelopes. When sampling different layers of the circumstellar envelope, observations of these molecular lines lead to detailed profiles of kinetic temperature, expansion velocity, and density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame radiative transfer code that predicts CO line intensities in the circumstellar envelopes of late-type stars is used in deriving relations between stellar and molecular-line parameters, on the one hand, and mass-loss rate, on the other. These expressions are applied to our extensive CO data set to estimate the mass-loss rates of 47 sample stars. Results: We present analytical expressions for estimating the mass-loss rates of evolved stellar objects for 8 rotational transitions of the CO molecule and thencompare our results to those of previous studies. Our expressions account for line saturation and resolving of the envelope, thereby allowing accurate determination of very high mass-loss rates. We argue that, for estimates based on a single rotational line, the CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss rates calculated for the asympotic giant branch stars range from 4 \times 10^-8 M_ȯ yr^-1 up to 8 \times 10^-5 M_ȯ yr^-1. For red supergiants they reach values between 2 \times 10^-7 M_ȯ yr^-1 and 3 \times 10^-4 M_ȯ yr^-1. The estimates for the set of CO transitions allow time variability to be identified in the mass-loss rate. Possible mass-loss-rate variability is traced for 7 of the sample stars. We find a clear relation between the pulsation periods of the asympotic giant branch stars and their derived mass-loss rates, with a levelling off at ~3 \times 10^-5 M_ȯ yr^-1 for periods exceeding 850 days. Conclusions: Appendices are only available in electronic form at http://www.aanda.org
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| 6. |
- Harwit, M., et al.
(författare)
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Polarisation observations of VY Canis Majoris H2O 532-441 620.701 GHz maser emission with HIFI
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L51-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Water vapour maser emission from evolved oxygen-rich stars remains poorly understood. Additional observations, including polarisation studies and simultaneous observation of different maser transitions may ultimately lead to greater insight. Aims. We have aimed to elucidate the nature and structure of the VY CMa water vapour masers in part by observationally testing a theoretical prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz maser components of ortho H2O. Methods. In its high-resolution mode (HRS) the Herschel Heterodyne Instrument for the Far Infrared (HIFI) offers a frequency resolution of 0.125 MHz, corresponding to a line-of-sight velocity of 0.06 km s(-1), which we employed to obtain the strength and linear polarisation of maser spikes in the spectrum of VY CMa at 620.701 GHz. Simultaneous ground based observations of the 22.235 GHz maser with the Max-Planck-Institut fur Radioastronomie 100-m telescope at Effelsberg, provided a ratio of 620.701 GHz to 22.235 GHz emission. Results. We report the first astronomical detection to date of H2O maser emission at 620.701 GHz. In VY CMa both the 620.701 and the 22.235 GHz polarisation are weak. At 620.701 GHz the maser peaks are superposed on what appears to be a broad emission component, jointly ejected from the star. We observed the 620.701 GHz emission at two epochs 21 days apart, both to measure the potential direction of linearly polarised maser components and to obtain a measure of the longevity of these components. Although we do not detect significant polarisation levels in the core of the line, they rise up to approximately 6% in its wings.
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| 7. |
- Menten, K. M., et al.
(författare)
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Herschel/HIFI deepens the circumstellar NH3 enigma
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L7-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Circumstellar envelopes (CSEs) of a variety of evolved stars have been found to contain ammonia (NH3) in amounts that exceed predictions from conventional chemical models by many orders of magnitude. Aims. The observations reported here were performed in order to better constrain the NH3 abundance in the CSEs of four, quite diverse, oxygen-rich stars using the NH3 ortho J(K) = 1(0)-0(0) ground-state line. Methods. We used the Heterodyne Instrument for the Far Infrared aboard Herschel to observe the NH3 J(K) = 1(0)-0(0) transition near 572.5 GHz, simultaneously with the ortho-H2O J(Ka,Kc) = 1(1,0)-1(0,1) transition, toward VY CMa, OH 26.5+0.6, IRC+10420, and IK Tau. We conducted non-LTE radiative transfer modeling with the goal to derive the NH3 abundance in these objects' CSEs. For the last two stars, Very Large Array imaging of NH3 radio-wavelength inversion lines were used to provide further constraints, particularly on the spatial extent of the NH3-emitting regions. Results. We find remarkably strong NH3 emission in all of our objects with the NH3 line intensities rivaling those of the ground state H2O line. The NH3 abundances relative to H-2 are very high and range from 2 x 10(-7) to 3 x 10(-6) for the objects we have studied. Conclusions. Our observations confirm and even deepen the circumstellar NH3 enigma. While our radiative transfer modeling does not yield satisfactory fits to the observed line profiles, it does lead to abundance estimates that confirm the very high values found in earlier studies. New ways to tackle this mystery will include further Herschel observations of more NH3 lines and imaging with the Expanded Very Large Array.
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| 8. |
- Cernicharo, J., et al.
(författare)
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Discovery of time variation of the intensity of molecular lines in IRC+10216 in the submillimeter and far-infrared domains
- 2014
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 796:1
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Tidskriftsartikel (refereegranskat)abstract
- We report on the discovery of strong intensity variations in the high rotational lines of abundant molecular species toward the archetypical circumstellar envelope of IRC+10216. The observations have been carried out with the Heterodyne Instrument for the Far-Infrared (HIFI) instrument on board Herschel and with the IRAM30 m telescope. They cover several observing periods spreading over three years. The line intensity variations for molecules produced in the external layers of the envelope most likely result from time variations in the infrared pumping rates. We analyze the main implications this discovery has on the interpretation of molecular line emission in the envelopes of Mira-type stars. Radiative transfer calculations must take into account both the time variability of infrared pumping and the possible variation of the dust and gas temperatures with stellar phase in order to reproduce the observation of molecular lines at different epochs. The effect of gas temperature variations with stellar phase could be particularly important for lines produced in the innermost regions of the envelope. Each layer of the circumstellar envelope sees the stellar light radiation with a different lag time (phase). Our results show that this effect must be included in the models. The submillimeter and far infrared lines of asymptotic giant branch stars can no longer be considered as safe intensity calibrators.
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| 9. |
- Ramstedt, Sofia, et al.
(författare)
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The wonderful complexity of the Mira AB system
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570, s. Art. no. L14-
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Tidskriftsartikel (refereegranskat)abstract
- We have mapped the (CO)-C-12(3-2) line emission around the Mira AB system at 0 ''.5 resolution using the Atacama Large Millimeter/submillimeter Array (ALMA). The CO map shows amazing complexity. The circumstellar gas has been shaped by different dynamical actors during the evolution of the system, and several morphological components can be identified. The companion is marginally resolved in continuum emission and is currently at 0 ''.487 +/- 0 ''.006 separation. In the main line component, centered on the stellar velocity, spiral arcs around Mira A are found. The spiral appears to be relatively flat and oriented in the orbital plane. An accretion wake behind the companion is clearly visible, and the projected arc separation is about 5 ''. In the blue wing of the line emission, offset from the main line, several large (similar to 5-10 '') opposing arcs are found. We tentatively suggest that this structure is created by the wind of Mira B blowing a bubble in the expanding envelope of Mira A.
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| 10. |
- Richards, A. M. S., et al.
(författare)
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ALMA sub-mm maser and dust distribution of VY Canis Majoris
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 572
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Cool, evolved stars have copious, enriched winds. Observations have so far not fully constrained models for the shaping and acceleration of these winds. We need to understand the dynamics better, from the pulsating stellar surface to similar to 10 stellar radii, where radiation pressure on dust is fully effective. Asymmetric nebulae around some red supergiants imply the action of additional forces. Methods. We retrieved ALMA Science Verification data providing images of sub-mm line and continuum emission from VY CMa. This enables us to locate water masers with milli-arcsec accuracy and to resolve the dusty continuum. Results. The 658, 321, and 325 GHz masers lie in irregular, thick shells at increasing distances from the centre of expansion. For the first time this is confirmed as the stellar position, coinciding with a compact peak offset to the NW of the brightest continuum emission. The maser shells overlap but avoid each other on scales of up to 10 au. Their distribution is broadly consistent with excitation models but the conditions and kinematics are complicated by wind collisions, clumping, and asymmetries.
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